The present invention relates to films comprising olefin/vinyl aromatic copolymer. The present invention relates especially to multilayer films containing ethylene/styrene copolymer.
The copolymerization of ethylene and styrene by a conventional Ziegler-Natta catalyst is reported in Polymer Bulletin, 20, p. 237 (1988). Ziegler-Natta catalytic methods are commonly used throughout the polymer industry, especially for the production of ethylene copolymers, and have a long history tracing back to about 1957. However, for an vinyl aromatic comonomer such as styrene the polymerization activity for such Ziegler-Natta catalysts is low, such that an ethylene/styrene copolymer has a maximum of only about 1 mole percent styrene units in the copolymer. Furthermore, because of the heterogeneity of conventional Ziegler-Natta catalysts, the reported copolymer is actually a mixture of polymer chains of varying length, some having more than 1% by mole of styrene per individual chain and many, if not most, having no styrene groups.
Conventional Ziegler-Natta catalysts are actually composed of many types of catalytic species, each at different metal oxidation states and different coordination environments with ligands. Examples of such heterogeneous systems include metal halides activated by an organometallic co-catalyst such as, for example, titanium or magnesium chlorides complexed to trialkyl aluminum. Because these systems contain more than one catalytic species, they possess polymerization sites with different activities and varying abilities to incorporate comonomer into a polymer chain. The result of such multi-site chemistry is a product with poor control of the polymer chain architecture both within the sequence of a single chain, as well as when compared to a neighboring chain. In addition, differences in catalyst efficiency produce polymers of high molecular weight at some sites and low molecular weight at others.
Recently, a new catalyst technology useful in the polymerization of polyolefins has been introduced. Examples of introductory articles include xe2x80x9cExxon Cites xe2x80x98Breakthroughxe2x80x99 in Olefins Polymerization,xe2x80x9d Modern Plastics, p.61, (July 1991); xe2x80x9cPolyolefins Gain Higher Performance from New Catalyst Technologies,xe2x80x9d Modern Plastics p.46, (October 1991); xe2x80x9cPW Technology Watch,xe2x80x9d Plastics World, p.29, (November 1991); and Plastics Technology, p.15, (November 1991). These polymerization systems are based on the chemistry of metallocenes, which are organometallic compounds which contain one or more cyclopentadienyl ligands attached to metals such as hafnium, titanium, vanadium, or zinconium. A co-catalyst, such as, but not limited to, oligomeric methyl alumoxane, is often used to promote the catalytic activity of the system. By varying the metal component and the cyclopentadienyl ligand, a diversity of polymer products may be tailored having molecular weights ranging from about 200 to greater than 1,000,000 and molecular weight distributions from 1.5 to about 15. The choice of co-catalyst influences the efficiency and thus the production rate, yield and cost.
The uniqueness of metallocene catalysts resides in the steric and electronic equivalence of each catalyst position. Specifically, metallocenes are characterized as having a single, stable chemical type rather than a volatile mixture of states as discussed above for conventional Ziegler-Natta catalysts. This results in a system composed of catalyst positions which have a singular activity and selectivity. For this reason, metallocene catalyst systems are often referred to as xe2x80x9csingle sitexe2x80x9d owing to their homogeneous nature. Polymers and copolymers produced by such systems are often referred to as single site resins by their suppliers.
In recent years several resin suppliers have been researching and developing metallocene catalyst technology, with the polymers produced thereby having a narrow molecular weight distributions.
Olefin/vinyl aromatic copolymers have been prepared using metallocene catalysts. These polymers have only up to 50 mole percent of the aromatic vinyl polymerization units, because the active site of the catalyst becomes crowded with the incorporation. of the sterically hindered aromatic vinyl comonomer, making it unlikely, or impossible, that another hindered comonomer could enter into the polymerization as the next monomer in the sequence.
These olefin/vinyl aromatic copolymers are similar in at lest some properties to other homogeneous, single site catalyzed copolymers such as the homogeneous, single site catalyzed ethylene alpha-olefins. That is, the present copolymers are characterized as having a narrow molecular weight distribution (MWD) and a narrow compositional distribution (CD). MWD refers to the breadth of the distribution of molecular weights of the polymer chains, and is a value which is obtained by dividing the number average molecular weight into the weight average molecular weight. The low CD, or regularity of side branch chains along a single chain and its parity in the distribution and length of all other chains, great reduces the low molecular weight and high molecular xe2x80x9ctailsxe2x80x9d. These features reduce extractables which arise from poor molecular weight control as well as improve optics by avoiding the formation of linear, ethylene-rich fractions.
However, at least in part because of the polarity of the vinyl aromatic comonomers used in the present invention, the present copolymers do not always follow the simplified trends of homogeneous, single site catalyzed ethylene alpha-olefins. Of course, it is the unique attributes of the present copolymers which make them desirable for use in a variety of film structures in accordance with the present invention. Yet, at least for those ethylene styrene copolymers which have a relatively low styrene content, generally from about 1% to about 10% by mole of styrene, processing and some physical properties are analogous to those of the homogeneous, single site catalyzed ethylene alpha-olefins. However, percent crystallinity drops much faster for the present copolymers than with any of the linear olefin comonomers of the homogeneous ethylene alpha-olefins. For the present copolymers 0% crystallinity occurs somewhere between 10% and 20% by mole of styrene. Thus, at least for those ethylene/styrene copolymers having between about 10% and 25% by mole of styrene, processing and some physical properties are analogous to known elastomers including the elastomeric homogeneous ethylene alpha-olefins. Yet, once again, a much greater percent by mole of alpha-olefin is required for homogeneous ethylene alpha-olefins to become elastomeric. Beyond 25% by mole of styrene such analogies to homogeneous ethylene alpha-olefins become less appropriate as the copolymer becomes more like polystyrene. Crystallinity generally remains at or about 0%; but, the glass transition temperature rises with an increase in amount of styrene sequences resulting in an increase in stiffness in resultant end products. However, the chemistry of the vinyl aromatic comonomer, whether it be styrene or a substituted styrene, is such that the present copolymers in each of the three comonomer content regions are beneficial when used in the production of appropriate film structure.
It has been discovered that olefin/vinyl aromatic copolymers provide stiffer amorphous regions, and therefore can be used to produce films having a high level of toughness, i.e., more resistant to puncture than, for example, ethylene/alpha-olefin copolymers. The stiffer amorphous regions provide advantages in films as applied force is dispersed more evenly, another reason films from such copolymers are tougher.
In the packaging arts, this result can be beneficial, for example, in that it could reduce or eliminate the need to use patch bags for the packaging of bone-in meat products.
Furthermore, the glass transition temperature (xe2x80x9cTgxe2x80x9d) of olefin/vinyl aromatic copolymers can be above room temperature, which means that these copolymers can be used to make films having improved lock-in of orientation at room temperature.
The chemical characteristics of olefin/vinyl aromatic copolymers, at the polymer level, provides a very large pendant group, These large pendant groups are effective in reducing the crystallinity of the polyethylene, because such large side groups cause very large crystalline defects. Olefin/vinyl aromatic copolymers can be used to make films-that are elastomeric in character, similar to other low crystallinity polyethylene. The olefin/vinyl aromatic copolymer have reduced molecular mobility resulting from the size of the aromatic pendant, the rigidity of the aromatic pendant, and the increased hydrogen bonding resulting from the unsaturation of the aromatic pendant. These attributes can be used to make a film having desirable stretch characteristics (i.e., low energy to stretch and a reduced tendency to yield, particularly when non-oriented). Such films are especially useful for in-store, central, and medical packaging films, where polyvinyl chloride films are currently used. Furthermore, the aromatic component can be utilized to make films exhibiting an improved level of crosslinking, as a result of the increased level of unsaturation in the aromatic component.
Furthermore, olefin/vinyl aromatic copolymers can be used to make films having good clarity, in combination with the high level of stiffness due to the aromatic component in the polymer, and the flexibility of the ethylene component in the polymer.
The chemical characteristics of olefin/vinyl aromatic copolymers, at the polymer level, can provide films comprising these copolymers with one or more of a wide variety of advantages. Olefin/vinyl aromatic copolymers have a relatively high void volume, which is related to the disruptive crystallinity, which can be used to make a film which is highly permeable with respect to gases, such as gaseous oxygen. Furthermore, the electronic structure of aromatic entities, such as styrene, can be used to permit xe2x80x9csolution diffusionxe2x80x9d of polar gases, such as gaseous oxygen, carbon dioxide, water vapor, etc., through a film containing such a composition. Such highly permeable films are useful in modified atmosphere packaging, especially dual-web packaging where an oxygen-impermeable film can be peeled off of the package in order to expose a gaseous-oxygen permeable package, so that meat in the package will bloom from exposure to gaseous oxygen passing through the film which is permeable to gaseous oxygen.
The high void volume in olefin/vinyl aromatic copolymers can also be used to provide films comprising these copolymers with the characteristic of increased stretch.
Furthermore, the high void volume of such copolymer, when used in films, can be used to make films having an increased capacity for absorption of relatively low molecular weight additives, such as antifog additives, antiblock additives, and antislip additives. Furthermore, the voids can be used to make films having the characteristic of taking up and retaining flavor additives, color additives, printing inks, etc., which can result in less contamination into the product within the package, or from one product to another. The property of differential absorption, when used to make printed films subjected to uv curing, can be used to result in a film effective to capture ultraviolet light in a film layer to be used as an activator used in the curing of the ink.
The high absorption characteristic can also be used to merely temporarily hold flavors, colors, etc., for films used in the packaging of a product wherein the flavor, color, etc., is thereafter transferred to the product, such as a cook-in turkey breast, etc.
The variety of mechanical and thermal transitions in olefin/vinyl aromatic copolymers can also provides films containing such copolymers with the characteristics having a broadened range of softening and DSC xe2x80x9cmeltingxe2x80x9d, which can be used to provide films with improved heat sealing characteristics. The wider variety of mechanical and thermal transitions in these copolymers can be further extended by blending such copolymers with additional polymers. Furthermore, because olefin/vinyl aromatic polymers have such a broad range of crystallization (the reverse of melting), these copolymers can interact with a broad range of other resins, and therefore are especially useful as tie layers in multilayer films, and can be used to reduce the net crystallization of mixtures of such resins. Furthermore, the resulting percentage increase in amorphous regions when used in a film, can be used to increase film properties such as shrink, tensile strength, elongation, and other related film characteristics.
The use of olefin/vinyl aromatic copolymers in films, both monolayer films as well as multilayer films, can provide one or more of a wide variety of improved properties to the film, and can be advantageous in certain end uses. For example, a film of improved printability can be prepared using monomers having the polarity of olefin/vinyl aromatic copolymers. Olefin/vinyl aromatic copolymers can be used: to provide a machinable film having a high gas transmission rate (especially advantageous in case-ready packaging in which a tray has a permeable layer overlaid with an oxygen barrier layer), with regard to both oxygen transmission rate as well as moisture transmission rate; to prepare films having a relatively low sealing temperature; to prepare films having improved optics through higher gloss and/or lower haze than is achieved from conventional alpha-olefin copolymers; in tie layers of multilayer films, to provide improved and diverse compatibility characteristics; to prepare films having improved resistance to tear propagation; to prepare films having improved sealability, in terms of improved hot tack, improved seal strength, and/or seal initiation temperature; to prepare films having improved impact strength; to prepare films having improved thermo-forming characteristics, such as deep draw characteristics; to provide films having improved organoleptic characteristics; to provide films having a lower level of extractables; to provide films having improved elastic recovery; to provide films having improved low-shrink characteristics; to provide films having improved wettability; to provide films having improved slip properties; to provide films having improved modulus; to provide films having improved stiffness; to provide films having improved softness; to provide films which are RF sealable; to provide films having improved stretch memory, high free shrink, greater toughness; to films having enhanced sealability after irradiation; to films having a desired glass transition temperature; to films having a selected Vicat Softening Point as a function of styrene content.
Olefin/vinyl aromatic copolymers can also be used in the preparation of foams, especially foam sheet, having improved thermoforming characteristics, especially in having improved resistance to cracking. The olefin/vinyl aromatic copolymer can be used to provide a polystyrene barrier tray, such as a barrier foam tray, with improved delamination resistance and structural integrity.
Olefin/vinyl aromatic copolymers can also be used to provide an improved patch for use on bag used in the packaging of bone-in meats.
As a first aspect, the present invention is directed to a film comprising a copolymer from a copolymerization of an alpha-olefin comonomer and a vinyl aromatic comonomer, wherein the alpha olefin comonomer comprises at least one member selected from the group consisting of C2 alpha-olefin, C3 alpha-olefin, C4 alpha-olefin, C5 alpha-olefin, C6 alpha-olefin, C7 alpha-olefin and C8 alpha-olefin, wherein the copolymer comprises alpha-olefin polymerization units in an amount of at least 50 mole percent.
Preferably, the alpha-olefin monomer comprises at least one member selected from the group consisting of ethylene and propylene. Preferably, the vinyl aromatic copolymer comprises styrene; more preferably, styrene.
At a second aspect, the present invention is directed to a film comprising composition comprising a thermoplastic, homogeneous alpha-olefin/vinyl aromatic copolymer. The homogeneous alpha-olefin/vinyl aromatic copolymer comprises recurring units of the formula: 
In the formula, xxe2x89xa7y, i.e., there must be a mole fraction of polymerization units derived from the alpha-olefin monomer (a mole fraction represented by xe2x80x9cxxe2x80x9d, as determined by x/(x+y)X 100) which is at least as great as the mole fraction of the polymerization units derived from the aromatic monomer (a mole fraction represented by xe2x80x9cyxe2x80x9d, as determined by y/(x+y)X 100). R1 is a member selected from the group consisting of hydrogen, C1 alkyl, C2 alkyl, C3 alkyl, C4 alkyl, C5 alkyl, C6 alkyl, and mixtures thereof; R2 is a member selected from the group consisting of hydrogen, C1 alkyl, C2 alkyl, C3 alkyl, C4 alkyl, C5 alkyl, C6 alkyl, and mixtures thereof; provided that R1 and R2 cannot both be alkyl. R3 is a member selected from the group consisting of hydrogen, aromatic, and mixtures thereof; R4 is a member selected from the group consisting of hydrogen, aromatic, and mixtures thereof; provided that one member, and only one member, selected from the group consisting of R3 and R4, is aromatic. Furthermore, a copolymer portion betweenn every two adjacent members selected from the group consisting of aromatic CHR3 and aromatic CHR4 comprises at least two xe2x80x94CH2xe2x80x94 units.
As a third aspect, the present invention is directed to a multilayer film comprising a first outer layer comprising polypropylene, a second outer layer comprising polypropylene, and an inner layer between the first outer layer and the second outer layer. The inner layer comprises a thermoplastic homogeneous alpha-olefin/vinyl aromatic copolymer as described immediately above in the first and second aspects of the present invention.
As a fourth aspect, the present invention is directed to a multilayer film comprising a first layer, a second layer, a third layer, and a fourth layer. The first layer is an outer layer, and serves as a heat sealing layer. The second layer is an inner (i.e., core) layer, and comprises the thermoplastic homogeneous alpha-olefin/vinyl aromatic copolymer as set forth above according to the first and second aspects of the present invention. The third layer is an inner layer and serves as a barrier to gaseous oxygen. The fourth layer is an outer layer.
As a fifth aspect, the present invention is directed to a multilayer film comprising a first layer and a second layer. The first layer serves as a barrier layer. The second layer serves as a heat sealing layer. The second layer comprises the thermoplastic homogeneous alpha-olefin/vinyl aromatic copolymer as set forth above in the first and second aspects of the present invention.
As a sixth aspect, the present invention is directed to a multilayer film comprising a first layer, a second layer, and a third layer. The first layer is an outer layer and comprises the thermoplastic homogeneous alpha-olefin/vinyl aromatic copolymer as set forth above in the first and second aspects of the present invention. The second layer is an inner layer. The third layer is an outer layer, and comprises a second thermoplastic homogeneous alpha-olefin/vinyl aromatic copolymer, which is a member of the group of homogeneous alpha-olefin/vinyl aromatic copolymers s set forth above in the first and second aspects of the present invention.
As a seventh aspect, the present invention is directed to a multilayer thermoformable article, comprising a first layer and a second layer. The first layer comprises a polystyrene web. The second layer comprises a multilayer, heat-resistant film, which, in turn, comprises the thermoplastic, homogeneous alpha-olefin/vinyl aromatic copolymer as set forth in the first and second aspects of the present invention.
As an eighth aspect, the present invention is directed to an RF sealable thermoplastic olefin-based film comprising thermoplastic, homogeneous alpha-olefin/vinyl aromatic copolymer comprising styrene polymerization units in an amount of from about 25 to 40 mole percent.
As a ninth aspect, the present invention is directed to a multilayer article comprising a heat-shrinkable patch member on a thermoplastic bag. The heat-shrinkable patch member comprises the thermoplastic homogeneous alpha-olefin/vinyl aromatic copolymer as set forth above in the first and second aspects of the present invention.